Question

I’m working through an algorithm problem set which poses the following question:

“Determine if a string has all unique characters. Assume you can only use arrays”.

I have a working solution, but I would like to see if there is anything better optimized in terms of time complexity. I do not want to use LINQ. Appreciate any help you can provide!

static void Main(string[] args)
{
    FindDupes("crocodile");
}

static string FindDupes(string text)
{
    if (text.Length == 0 || text.Length > 256)
    {
        Console.WriteLine("String is either empty or too long");
    }

    char[] str = new char[text.Length];
    char[] output = new char[text.Length];

    int strLength = 0;
    int outputLength = 0;

    foreach (char value in text)
    {
        bool dupe = false;
        for (int i = 0; i < strLength; i++)
        {
            if (value == str[i])
            {
                dupe = true;
                break;
            }
        }
        if (!dupe)
        {
            str[strLength] = value;
            strLength++;

            output[outputLength] = value;
            outputLength++;
        }
    }
    return new string(output, 0, outputLength);
}

Answer 1

If time complexity is all you care about you could map the characters to int values, then have an array of bool values which remember if you’ve seen a particular character value previously.

Something like … [not tested]

bool[] array = new bool[256]; // or larger for Unicode

foreach (char value in text)
  if (array[(int)value])
    return false;
  else
    array[(int)value] = true;

return true;